A solar cell (or photovoltaic cell) is a semiconductor device that converts photons into electricity. Solar cells generate charge carriers (electrons and holes) in a light absorbing material. Solar cells separate the charge carriers to conductive contacts that transmit electricity. This conversion is called the photovoltaic effect, and the field of research related to solar cells is known as photovoltaics. A solar panel is a collection or arrangement of solar cells. The arrangement of solar cells may be in an array.
Solar cells may be grouped together to form arrays and an array or arrays may be arranged as desired on a solar panel. The panels, in turn, may be arranged in an array. Dust settling on the solar cells obscures the light and therefore reduces the amount of electricity that the solar cells produce. The problem exists in terrestrial applications of solar cells as well as in applications on other planets (extra terrestrial applications).
The article entitled “An Optical System for the Quantitative Study of Particulate Contamination on Solar Collector Surfaces” by S. Biryukov, D. Faiman and A. Goldfeld, Solar Energy, Vol. 66, No. 5 pp. 371-378, 1999, and the article “An Experimental Study of the Dry Deposition Mechanism for Airborne Dust” by S. Biryukov, Journal of Aerosol Sciences, Vol. 29 No. 1/2 pp. 129-139, 1998, indicates that there is 10% degradation of solar cell performance on earth caused by dust. The article describes a study of dust settlement in a desert area and a system to measure dust deposition with the goal of development of methods for retarding the settlement of dust and self cleaning of dust that has settled. The papers indicate that there is a need for self cleaning of dust from solar cells as it has advantages over cleaning the solar cells for example by washing them with water.
Solar cells have been used and are planned to be used in space missions to other planets. Solar cell arrays are powering two rovers sent to Mars. These rovers suffered from degradation of their power supply system because of dust that covered their solar arrays.
The Abstract of an Article Entitled
“Mars Dust Removal Technology” by Geoffrey A. Landis, Ohio Aerospace Institute, Cleveland, Ohio 44135, published as paper IECEC 97-97345 in the proceedings of the Energy Conversion Engineering conference, Jul. 27-Aug. 1, 1997 (also published in AIAA Journal of Propulsion and Power, Vol. 14, No. 1, pp. 126-128, January 1998) states: “The Mars atmosphere contains a significant load of suspended dust. Settling of atmospheric dust onto the surface of the solar array is potentially a lifetime-limiting factor for a power system on any Mars mission. For long-term operation of [solar] arrays on Mars, it may be necessary to develop techniques to remove deposited dust. Dust is expected to adhere to the array by Van der Waals adhesive forces. These forces are quite strong at the dust particle sizes expected. If the array surface is insulating, it is possible that they may also be subject to electrostatic adhesion, which may be extremely strong. Dust-removal methods must overcome this force. Dust-removal methods can be categorized briefly into four categories: natural, mechanical, electromechanical, and electrostatic. The environment of Mars is expected to be an ideal one for use of electrostatic dust-removal techniques.”
Dr. Landis further states in the article that: “The atmosphere of Mars is known to contain a significant load of suspended dust. This atmospheric dust will have several effects on the use of photovoltaic power systems on the surface, including decreasing the amount of sunlight on the surface and shifting the spectrum of the available sunlight. There are several mechanisms for sand and dust to be deposited on a solar array. Particles may accumulate on the array by the process of saltation, the lifting of particles by wind. On Mars, the particle sizes most easily lifted by wind have a range of 50-100 μm in radius, and hence is best referred to [as] fine sand. The trajectories average a height of 10 to 20 cm off the surface. At the Mars atmospheric pressure of about 10 mbar, saltation occurs at wind velocities over about 15 m/s, a wind velocity only seen during brief gusts at the Viking lander sites. To avoid coverage of the array by saltation, it is desirable to design the arrays to be at least 20 cm from the surface.
It is expected that atmospheric dust will settle out of the atmosphere. The rate and mechanism of settling are not well characterized, but estimates indicate that obscuration of an array surface by dust may cause between 22% and 89% degradation in performance over the course of a two-year mission. This is shown in Table 1.
Some information on the settling of dust has been generated by the Materials Adherence Experiment on the Mars Pathfinder mission, which shows a deposition rate of about 0.3% coverage per day . . . but the deposition rate is expected to be both geographically variable and also to vary from season to season and from year to year.
obscurationobscurationCase(30 day mission)(2 yr mission)Baseline 6.6%77%Best 0.5%22%Worst52.2%89%Table 1: Calculated power loss of solar array due to settled dust for 30-day and 2-year missions on Mars for baseline, best-case, and worst-case scenarios.” (Citations Omitted)
The 1997 article by Dr. Landis just quoted in detail above further indicates that solar cells (photovoltaic cells) may “conceivably” be cleaned by glow discharge cleaning. But no details are given as to how this technique is to be used. It is understood that the glow discharge is used as a means to charge the dust with electrostatic charge in order to repel it. The dust removal is not accomplished by a plasma generated “wind.”
A technique to remove dust by generating a moving electrostatic wave which discloses a transparent dust shield having parallel electrodes etched on a clad board driven by an alternating current source is described in the article “Development of a Transparent Self-Cleaning Dust Shield for Solar Panels” by Sims, R. A., Biris, A. S., Wilson, J. D., Yurteri, C. U., Mazumder, M. K., Calle, C. I., and Buhler, C. R., Proceedings of the ESA-IEEE Joint Meeting on Electrostatics 2003, Laplacian Press, Morgan Hill, Calif., pp. 814-821 (2003). Various line thicknesses and spacings were used. The electrodes, as understood, were covered by a highly resistive polyurethane coating to prevent discharge between the electrodes at high voltages. The lines were arranged in parallel such that every other line was connected to one terminal of an alternating current source. As understood this article is reporting and disclosing use and testing of electrodes on the same side of a substrate or screen. As understood, this arrangement works on an electrostatic wave principle and is not based upon a plasma generated “wind.” The reference discloses operation at a frequency between 0-300 Hz while the voltage was varied from 0-10 kV. The reference appears to generally conclude that cleaning of the screen is increased at high frequency and high voltage. The article discloses that the use of the pulsed-wave was particularly effective in obtaining a high clearing factor. The next-most effective wave form was the square wave followed by the sinusoidal wave. Triangularly shaped waves were also applied but their efficacy in regard to clearing factor was not reported.
Another reference entitled “Electrohydrodynamic Force and Acceleration in Surfaces Discharges,” J. Boeuf, Y. Lagmich, T. Calligari, and L. Pitchford, CPAT, CNR, Université P. Sabatier, Toulouse, France, AIAA-2006-3574, 37th AIAA Plasmadynamics and Lasers Conference, San Francisco, Calif., Jun. 5-8, 2006 discloses the basic electrode relationship for a dielectric barrier discharge device and concludes that the volume of the region where the ElectroHydroDynamic (EHD) force takes place is strongly dependent on the voltage risetime rate (or frequency, for a sinusoidal voltage). A clean signal is not required but fast risetime is advantageous for efficient functioning of a dielectric barrier discharge device. As understood, the force is distributed over a larger volume when the voltage increase rate is smaller. This article sets forth a numerical estimation for the force exerted on the gas molecules ahead of the ion sheath ahead of the plasma. In the reference, the force exerted on the discharge on the molecules is a function of the sheath length divided by the sheath velocity times the potential across the actuator squared divided by the sheath length cubed times the permittivity of free space.
FIG. 1 is a schematic of an existing dielectric barrier discharge device 100 having a bottom electrode 101, a top electrode 102 and a dielectric substrate 105 between the electrodes. Both electrodes 101 and 102 are affixed to the dielectric by adhesive 103, 104. Plasma 106 is illustrated forward of the top electrode with an ion sheath 106A leading the plasma. An alternating current source 107 is illustrated as being applied across the electrodes 101, 102.
Collisions between the ions and the neutrals creates the induced flow represented by the arrow and reference numeral 108. The induced flow occurs because the plasma generates a body force that creates velocity or a “wind” of gas. By “gas,” it is meant the atmosphere in which the device resides.
Devices similar to the one described in FIG. 1 and variations of the device are used in aerodynamic applications. The idea is to affect external flow around airfoils and bodies and internal flow in inlets, ducts and turbomachinery passages for the purpose of achieving control of the flow and to reduce acoustic noise. The approach is called active flow control. The device can introduce momentum or disturbances into the flow that in turn have been shown in laboratory experiments to eliminate and reduce flow separation and control circulation around airfoils. Active flow control can be achieved by other types of devices, mainly pneumatic type devices that inject fluid into the flow or suck fluid out of the flow. However plasma devices discussed here have advantages over the pneumatic devices that make them attractive.
The article entitled “Electrohydrodynamic Flow Control with a Glow-discharge Surface Plasma” by J. R. Roth, D. M. Sherman and S. P. Wilkinson, AIAA Journal Vol. 38, No. 7, July 2000, describes experiments aimed mainly at drag reduction.
The article entitled “Overview of Plasma Flow Control: Concepts, Optimization and Application” by T. Corke and M. Post, AIAA Paper 2005-0563, 2005, reviews several applications in aerodynamics mainly aimed at separation elimination and circulation control.
U.S. Pat. No. 6,200,539 to Roth et al discloses a substrate configured with first and second sets of electrodes, where the second set of electrodes is positioned asymmetrically between the first set of electrodes. When an RF (radio frequency) voltage is applied to the electrodes sufficient to generate a discharge plasma (e.g. as named by Mr. Roth, “one-atmosphere uniform glow discharge plasma”) in the gas adjacent to the substrate, the asymmetry in the electrode configuration results in force being applied to the active species in the plasma and in turn to the neutral background gas. It appears that the '539 patent to Roth operates at voltages greater than 1 kV.
U.S. Pat. No. 5,938,854 to Roth discloses a method and apparatus for cleaning surfaces with a glow discharge plasma at one atmosphere of pressure. Reference is made to FIG. 5d of the '854 patent to Roth wherein it is indicated that electrodes are embedded in an insulating coating to create plasma above electric field lines. This arrangement does not create a “wind”.
A reference entitled “Demonstration of Separation Delay With Glow-Discharge Plasma Actuators,” Lennart S. Hultgren and David E. Ashpis, NASA-TM-2003-2122041, REV 1, (AIAA-2003-1025), Glenn Research Center, Cleveland, Ohio, December 2004, discloses an actuator which is fabricated from printed-circuit board technology using seven electrode pairs. Application of a time-varying voltage to the electrode pairs is illustrated. This reference does not address the solar panel application. It does describe sequential applied voltage pulsing. This reference does not disclose the use of one pair of electrodes to create two plasma filled regions.
The article entitled “Aerodynamic flow acceleration using paraelectric and peristaltic electrohydrodynamic effects of a One Atmosphere Uniform Glow Discharge Plasma” by R. Roth, Physics of Plasmas Vol. 10, No. 5, pp. 2117-2126 May 2003, describes arrays of electrodes and phased operation.
The paper “Modeling of interaction between weakly ionized near-surface plasmas and gas flow” by A. V. Likhanskii, M. N. Shneider, S. O. Macheret and R. B. Miles, AIAA paper 2006-1204 is a numerical simulation of the device studying its operation under two types of voltage input. The first is the sinusoidal AC input at RF (Radio Frequency). The second is ultrashort pulses of a few nanosecond pulse width at high repetition rate of hundreds of KHz in combination with a positive DC voltage bias.
Geometry of electrodes and other characteristics were studied in the paper entitled “Optimization of the Aerodynamic Plasma Actuator as an Electrohydrodynamic (EHD) Electrical Device” by J. R. Roth and X. Dai, AIAA paper 2006-1203, January 2006. Different geometries of electrodes were studied and their operation under AC voltage and frequencies in the RF range to characterize performance with different dielectric material and amount of power dissipated to heat to determine their efficiency.
Another reference entitled “Optimization of a Dielectric Barrier Discharge Actuator by Stationary and Non-stationary Measurements of the Induced Flow Velocity—Application to Airflow Control” by M. Forte, J. Jolibois, E. Moreau and G. Touchard, AIAA paper 2006-2863, 2006, describes characteristics of the devices.
Plasmas are conductive assemblies of charged particles, neutrals and fields that exhibit collective effects. Further, plasmas carry electrical currents and generate magnetic fields. Plasmas are the most common form of matter, comprising more than 99% of the visible universe, and permeate the solar system, interstellar and intergalactic environments. Sometimes plasmas are designated by scientists as the fourth state of matter with gas, solid, and liquids being the other states of matter.
The terms glow discharge plasma and dielectric barrier discharge are terms used by those skilled in the art. Nonetheless, those terms are sometimes used imprecisely.
Historically, glow discharge was associated with DC discharge in vacuum. Later on there was an interest in AC discharges and discharges at atmospheric pressures. A dielectric barrier was added to the electrodes. The advantage is that the dielectric barrier with AC operation serves as a self-limiter allowing maintenance of a low power stable discharge without undesirable arcing that generates heat and large current and is not low power. The self-limiting mechanism is attributed to surface charges that accumulate on the dielectric surface and oppose the electrical field and result in repetitive stop and start of the discharge as the AC voltage progresses, limiting the current and preventing transition to the arc regime. The correct term for the phenomena is dielectric barrier discharge (DBD), but sometimes the term glow discharge is used incorrectly. The term single dielectric barrier discharge (SDBD) is also sometimes used to indicate that there is one layer of dielectric.
Electric glow discharges are found in a variety of areas, including lighting (fluorescent lights), television (plasma-screen television), plasma physics, material processing and analytical chemistry. In science, glow discharges are most often operated in direct-current mode. The electrical potential, gas pressure, and electrical current are interrelated. Only two can be directly controlled at once, while the third must be allowed to vary. Typically, the pressure is held constant. The other constant parameter depends on the application.
Wikipedia provides the following description: “A nonthermal plasma is, in general, any plasma or weakly ionized gas which is not in thermodynamic equilibrium, either because the ion temperature is different from the electron temperature, or because the velocity distribution of one of the species does not follow a Maxwell-Boltzmann distribution . . . . The nomenclature for nonthermal plasma found in the scientific literature is varied. In some cases, the plasma is referred to by the specific technology and configuration used to generate it (“gliding arc”, “plasma needle”, “plasma jet”, “resistive barrier discharge”, dielectric barrier discharges etc.), while other names are more generally descriptive, based on the characteristics of the plasma generated (“one atmosphere uniform glow discharge plasma”, “atmospheric plasma”, “ambient pressure nonthermal discharges”, “non-equilibrium atmospheric pressure plasmas”, etc.)“. See, http://en.wikipedia.org/wiki/Nonthermal_plasma.
Dielectric-barrier discharges (DBD's) comprise a specific class of high-voltage, AC, discharges. Their defining feature is the presence of charges on the dielectric layers that limit the current.